1. Technical Field
The invention relates to a chip resistor which is widely used in an electronic circuit, particularly to a chip resistor which has a low resistance and a low TCR, and also to a method of producing the resistor.
2. Background Art
Recently, as typically exemplified by a portable telephone, a movie camera, and a notebook-type personal computer, demands for small electronic apparatuses are growing. It is no exaggeration that miniaturization and improvement of the performance of such electronic apparatuses will depend on those of chip-type electronic parts to be used in the apparatuses. As a thin film resistor body, known are ruthenium oxide and a composition which contains bismuth ruthenate and lead ruthenate that are complex oxides of ruthenium oxide, as main components (for example, see the Unexamined Japanese Patent Application Publication No. Sho 58-37963). Such a resistor body is used in various fields.
An example of a method of producing a conventional chip resistor will be described with reference to the accompanying drawings. FIG. 12 is a perspective view showing an example of the structure of a conventional chip resistor, and FIG. 13 is a section view taken along the line A-A' of FIG. 12. Usually, a chip resistor of this kind is produced in the following manner. First, upper electrodes 11 are formed on the upper face of a chip-like alumina substrate 10 which is made of alumina of 96% purity. A resistor body 12 is formed on a part of the upper face of the alumina substrate 10 so as to be connected with the upper electrodes. A protective film 14 which is made of lead borosilicate glass is formed so as to cover the whole of the resistor body 12. Usually, the protective film 14 is formed by forming a pat-tern by means of screen printing and then firing the film at a temperature as high as 500 to 800.degree. C.
Next, end-face electrodes 13 each consisting of an Ag thick film are formed on the end faces of the alumina substrate 10 so as to be connected with the upper electrodes 11, respectively. Usually, the end-face electrodes 13 are formed by conducting a firing process at a high temperature of about 600.degree. C. In order to ensure the reliability in a soldering process, finally, Ni plated films 15 are formed by electroplating so as to cover the end-face electrodes 13, and solder plated films 16 are formed so as to cover the Ni plated films 15, thereby completing a chip resistor.
In a chip resistor produced by such a production method, generally, a thick film glaze resistor body material which contains ruthenium oxide as a main component is used as conductive particles constituting the resistor body. However, a resistor body material which contains only ruthenium oxide has a large temperature coefficient of resistance (hereinafter, often abbreviated as "TCR") which indicates a change of the resistance with temperature. Therefore, the material must be used after the TCR is reduced to a small value of about .+-.50 ppm/.degree. C. or less by adding a TCR adjustment material such as a metal oxide.
When such a resistor body material is used, however, it is difficult to produce a chip resistor having a low resistance of 1 .OMEGA. or less because ruthenium oxide has high resistivity. To comply with this, a chip resistor has been proposed in which a copper nickel alloy having a low temperature coefficient of resistance, such as that described in JIS C2521 and JIS C2532 is used as a resistor body material of a low resistance of 1 .OMEGA. or lower.
Specifically, a structure is proposed in which such an alloy material is formed into a foil-like or plate-like shape and then applied to an alumina substrate, and that in which resistor body paste obtained by kneading copper powder, nickel powder, and a glass frit in an organic vehicle is printed on an alumina substrate and then fired in an inert atmosphere, thereby forming an alloy film (see the Unexamined Japanese Patent Application Publication Nos. Hei 2-308501 and Hei 3-270104).
In the former structure, however, the mass productivity is not highly excellent because of the following reason. Under the situation where miniaturization of a chip part is growing, a method of working alloy foil or an alloy plate has a limit, a trimming process cannot use a laser, and other processes such as grinding have a limit. Furthermore, also from the view point of cost, the method is more disadvantageous than the printing method.
In the latter structure, the bonding between the resistor body film and the substrate, and the adjustment of the resistance layer are realized by using glass, and hence components other than copper-nickel are contained at high ratios. Consequently, the temperature coefficient is different from that of a copper nickel alloy. Depending on the firing conditions, furthermore, the glass component exhibits diffusion behavior in the metal components and at the interface between sintered particles in different manners. Therefore, the latter structure has a problem in that a stable resistance property is hardly obtained.
In the paste method using copper powder and nickel powder, the properties of a resistor are largely affected by the properties of terminal electrodes of a power supply portion, and the structure of the interface between the resistor body and an electrode. The minimum resistance which can be produced by the method is limited to 100 m.OMEGA.. It is difficult to realize a lower resistance.
As described above, the recent tendency to miniaturization of a chip resistor is growing. On the other hand, the needs for a chip resistor which may be used in current detection in an electronic circuit, and the like and which has a low resistance and a low TCR is increasing. From the view point of the performance required in a use, moreover, a chip resistor which can ensure high accuracy and high reliability in addition to a low resistivity and a low TCR is eagerly requested.